The air squat - such a simple idea and yet its execution is always riddled with problems.

A few simple cues to remember: chest up, weight on heels, knees and feet in line with each other, tight 'core'. If you have the dreaded 'but wink' grab a bar and practice getting into the bottom squat position. If your knees are coming out over your toes, go to a wall and do some 'wall seductions', ie, place your arms over your head and work on keeping the knees from touching the wall.